> or will the 4:3 resolution be too bad when used like this? (not running in native mode)actually at the 4:3 resolution you will still be running in the lcd's native resolution (providing you set mame to not allow switching from your native resolution of course, and have windows set to your lcd screen native resolution to begin with). it will just be a different aspect ratio (ie. 4:3)
so, if u are running the lcd in its native resolution, then the picture will be sharp and clear both in 4:3 and 16:9 mode, so there wont be any problems there (the only difference will be in 4:3 mode you will have the black borders at the top and bottom, which you are not worried about anyway, so thats ok)
as for running horizontal games on a vertically mounted screen, i suppose the question would be if you will see any visible 'uneven stretching' of the image. im not sure of that answer but hopefully you wont, just make sure the 22" lcd monitor you are purchasing has a high native resolution (1920x1080 would be ideal

) .. but perhaps lower would be ok also (eg. 720p monitor), dont quote me on that though, i havnt done any experiments myself..